Organic electronic elements are elements intended for electrical operations with the use of organic matters, expected to be able to provide features such as energy conservation, low prices, and flexibility, and attracting attention as alternative techniques to conventional inorganic semiconductors mainly containing silicon.
Examples of the organic electronic elements include organic EL elements, organic photoelectric conversion elements, and organic transistors.
Among the organic electronic elements, the organic EL elements are attracting attention, for example, as alternatives to incandescent lamps and gas-filled lamps, and for use as large-area solid-state light sources. In addition, the organic EL elements are also attracting attention as most likely self-luminous displays in place of liquid crystal displays (LCD) in the field of flat panel display (FPD), and increasingly put into production.
In recent years, for the purpose of improving the organic EL elements in luminescent efficiency and lifetime, attempts have been made to use a charge transporting compound mixed with an electron-accepting compound.
For example, Patent Literature 1 discloses a composition composed of an ionic compound and a charge transporting compound, as a composition for charge transporting films.
However, these materials fail to sufficiently achieve effects such as reductions in driving voltage and improvements in luminescent efficiency and lifetime. Furthermore, the materials are low in thermal stability, deteriorated by baking in the preparation of organic EL elements or by driving organic EL elements, and the organic EL elements have problems of decreases in luminescent efficiency and lifetime and variations in characteristics of the organic EL elements. Moreover, there has been a problem that decomposition products resulting from the electron accepting compound in baking in the preparation of elements damage organic EL manufacturing devices, etc. to decrease the productivity.
On the other hand, the organic EL elements are classified roughly into two types of: low molecular weight-type organic EL elements and high molecular weight-type organic EL elements, according to materials and film forming methods used. The high molecular weight-type organic EL elements are essential elements to large-screen organic EL displays in the future, because organic materials are composed of high molecular weight materials, and able to be easily formed by printing, ink jet printing, etc., as compared with the low molecular weight-type organic EL elements which require film formation in a vacuum system.
Both the low molecular weight-type organic EL elements and high molecular weight-type organic EL elements have been energetically researched, but still have the significant problem of being low in luminescent efficiency and short in element lifetime. As one means for solving this problem, multi-layered elements have been attempted for the low molecular weight-type organic EL elements.
FIG. 1 shows an example of a multi-layered organic EL element. In association with FIG. 1, a layer in charge of light emission is referred to as a light emitting layer 1, and in the case of including other layers, a layer in contact with an anode 2 is referred to as a hole injecting layer 3, and a layer in contact with a cathode 4 is referred to as an electron injecting layer 5. Furthermore, when there is a distinct layer between the light emitting layer 1 and the hole injecting layer 3, the distinct layer is referred to as a hole transporting layer 6, and furthermore, when there is a distinct layer between the light emitting layer 1 and the electron injecting layer 5, the distinct layer is referred to as an electron transporting layer 7. It is to be noted that reference numeral 8 denotes a substrate in FIG. 1.
For the low molecular weight-type organic EL elements, films are formed by a vapor deposition method, and multi-layered elements can be thus easily achieved by carrying out vapor deposition while sequentially changing compounds used. On the other hand, for the high molecular weight-type organic EL elements, films are formed with the use of a wet process such as printing or ink-jet printing, and a problem is thus caused which is that the lower layer is dissolved when the upper layer is applied. Therefore, it is difficult to achieve multi-layered high molecular weight-type organic EL elements, as compared with the low molecular weight-type organic EL elements, and it has not been possible to achieve the effect of improving the luminescent efficiency or improving the lifetime.
In order to address this problem, several methods have been ever proposed. One of the methods is a method of using a difference in solubility. For example, there is an element that has a two-layer structure of: a hole-injecting layer of water-soluble polythiophene:poly(styrene sulfonate) (PEDOT:PSS); and a light emitting layer formed with the use of an aromatic organic solvent such as toluene. In this case, the PEDOT:PSS layer is not dissolved in the aromatic solvent such as toluene, and it is thus possible to prepare the two-layer structure.
Patent Literature 2 discloses an element of three-layer structure, which has a layer referred to as an interlayer layer introduced on PEDOT:PSS.
These methods for multi-layers are important, but problematic in that the use of water-soluble PEDOT:PSS requires baking for a long period of time because of the need to remove water remaining in thin films, and that evaporated water damages manufacturing devices. In addition, there have been manufacturing problems such as the need for heating at high temperature for insolubilization of hole transporting layers, thus decreases in lifetime and luminescent efficiency of the organic EL elements due to material deterioration by the heating, and a step of rinsing dissolved matters for inadequate changes in solubility.